I learned two so far and mastered one of them. But it seems i can't learn any more. I can't use them either?

Techniques seem to be per-selection-of-weapons rather than an explicit per-weapon-category, but I don't get what you mean by "can't learn any more". If you find a bookshelf with a new technique it should go into your list, and if you should pull off the technique before learning it from a book, it should also be added. I think? That's how it's seemed to work for me, anyway.

I thought there was some sort of mechanic I hadn't yet found for mastering techniques, because the mastery bars on all of them are completely bottomed out despite that I perform some of them pretty regularly.

The mastery may be linked to the enemies you defeat with the attack, I haven't tried it that much but one of the newer ones I found is much higher than my first one. This could be due to using that weapon more, but I know I killed one of the minibosses with a special attack.

I feel like a similar system was used in Portrait of Ruin, which upgraded based on the XP from the enemy killed by the attack. In other words, using it to 'hurt' enemies would never advance it's level, but beating them down and using the special for the final blow, you could rake in the XP. This seems to be the case until you perform that special attack. Once you read it, you can check back in your archive or whatever it's called. If you have one of those weapons, performing the attack should add it to your techniques, randomly finding the special attack should do the same.

Only certain weapons can perform certain techniques. You don't have to unlock them from the bookshelves, though, simply performing the correct technique with the correct weapon will "unlock" it.

To use the technique with a DIFFERENT WEAPON FROM THE SAME CATEGORY, that does NOT have the technique to begin with, you need to master it, i.e. hitting monsters with the technique until the mastery bar is full. You can then use the technique on all weapons of the same category, regardless whether it had it or not.

Example: The Nodachi has a block move (Forward, Forward + Attack). The Tsurumaru has a forward slash attack (Down, Down-Forward, Forward + Attack). Mastering the Block move from the Nodachi means you can now use it with the Tsurumaru equipped, while mastering the forward slash would allow you to use that with the Nodachi.

Different weapon types have a different amount of techniques to unlock and master (Katanas have 5, Greatswords on the other hand seem to have only 2).

I'm trying to use Thousand Blossoms but it doesn't work. :(
I'm trying with the Rapier which seems to be one of the weapons it works for.